L-Rhamnose, which is a typical 6-deoxyhexose similar to fucose and has a CH.sub.3 group, is also called 6-deoxy-L-mannose, L-mannomethylose or isodulcitol. L-Rhamnose is usually obtained in the form of .alpha.-type monohydrate crystal from its aqueous solution. This crystal has a melting point of 88.degree. to 92.degree. C. and is sublimable. The specific rotation of an aqueous solution thereof is initially levorotatory ([.alpha.].sub.D.sup.20 =-7.7.degree.) but it undergoes mutarotation to become dextrorotatory ([.alpha.].sub.D.sup.20 =ca.+9.degree.) within approximately one hour.
L-Rhamnose exhibits a sweet and slightly bitter taste similar to that of D-mannose.
Recently it has been found that sugar chains comprising various monosaccharides have physiological activities. Thus there have been attempts to utilize these sugar chains as materials for pharmaceuticals and agricultural chemicals. L-Rhamnose or its derivatives are sometimes employed as the constituents of these sugar chains. Further it would be expected that L-rhamnose is available in the field of biotechnology including plant cytology, microbial technology, genetic engineering, fermentation technology and immunology. L-Rhamnose would be furthermore expected to be available as a material for a reaction flavor since it would undergo the Maillard reaction with an amino acid to thereby generate a characteristic smell.
L-Rhamnose widely occurs in nature, mainly in plants, as a constituent of glycosides such as rutin, xanthorhamnin, hesperidin or quercitrin, or a constituting sugar of, for example, gum arabic or karaya gum.
L-Rhamnose has been prepared by hydrolyzing the above-mentioned glycosides. However these glycosides are expensive and produced only in a limited amount, which makes the use thereof as the starting materials for the preparation of L-rhamnose difficult. In addition, the use of, for example, rutin is sometimes undesirable since it might be contaminated with quercetin which might be caroinogenic.
Japanese Patent Laid-Open No. 146200/1986 discloses a process for preparing L-rhamnose from gum arabic. This process requires use of a large amount of organic solvent(s) for the removal of impurities and for the separation of the aimed L-rhamnose from L-arabinose which is contained in the reaction mixture in the same amount as that of the L-rhamnose. Further an industrial chromatographic separation in this process requires a troublesome procedure as well as well-controlled equipment. Thus it is difficult to economically prepare L-rhamnose thereby.
As described above, it is very difficult at present to economically and stably supply L-rhamnose on an industrial scale. That is to say, when an expensive material produced in a limited amount, e.g., rutin is employed, the L-rhamnose thus obtained is also expensive. On the other hand, when gum arabic is employed as a material, the resulting sugar hydrolyzate has an undesirable composition, which makes it necessary to employ special equipment for, e.g., chromatographic separation to obtain L-rhamnose of a high purity.